Work Heat Pump
ABSTRACT
The analysis of 3 different plants was carried out in the lab, energy supplied and work output was measured for all systems, equations derived in class were used to find the efficiencies
1.INTRODUCTION
The labs involved systems from the heat pump and refrigeration cycles, the vapour power cycles and reciprocating internal combustion Engines which were extensively covered in the course. In a power cycle, heat is received by the working fluid at a high temperature and rejected at a low temperature, while the net amount of work is done by the fluid. In the vapour power cycle the method of producing mechanical power employs the transfer of heat from a reservoir to a working fluid which is taken through a thermodynamic cycle, in these two systems there is a relationship between fluid properties and the quantities of work and heat which accompany changes of states , therefore it is possible to design a plant which will effect the desired changes of state. However with a internal combustion engine whose main purpose is the transfer of work and for exchanges of heat with the surroundings are only secondary
2. OBJECTIVES
To measure Brake Power, Specific Fuel Consumption and Brake Thermal Efficiency as functions of engine speed
To measure boiler, turbine and steam power plant efficiency
To determine the Coefficient of Performance for a mechanical heat pump and a Refrigerant
3 EXPERIMENTAL WORK
The coursework consisted of 3 labs, a Combined Refrigeration and Mechanical Heat Pump system, a Boiler, Turbine and Steam Power Plant Efficiency Measurement and a 1500cc, 4cyl Diesel Engine Test
3.1 Description of Apparatus
Combined Refrigeration and Mechanical Heat Pump system
The apparatus consists of a heat pump and a refrigerant. the refrigeration cycle is an efficient provider of heat as well as cooling, this principle is used in the experiment. There are two principle locations in the transfer of heat; the place where heat is absorbed, (Evaporator tank), and where it is rejected, (Condensing tank). The compressor in the refrigeration system also produces waste heat, and fundamentally a significant proportion of this can be recovered, however a fan is used to cool it down.
The mechanical refrigeration cycle consists of an arrangement of heat exchangers; one that absorbs heat, the other that rejects it. This heat absorbed is transported through a sealed system of pipes by a fluid, the refrigerant, circulated by a compressor. The refrigerant is a fluid that has a low boiling point.
In order to absorb and release the heat into and from the refrigerant, we exploit the ability of the refrigerant fluid to boil from a liquid to a vapour and then to condense back into a liquid. This is a continual process while the compressor is running and circulating the refrigerant
1500cc, 4cyl Diesel Engine Test
This apparatus is simply a diesel engine lifted from a road vehicle. The defining feature of an internal combustion engine is that useful work is performed by the expanding fluids acting directly to cause movement of the pistons which creates mechanical power at the crankshaft. In this experiment we measure the efficiencies of the ICE- The brake power is historically measured by a brake at the engine output shaft, the angular velocity is measured from the output shaft and a reactant torque is applied from a dynamometer, subsequent to this the specific fuel consumption and the brake thermal efficiency is measure as a function of engine speed
Boiler, Turbine & Steam Power Plant Efficiency Measurement
The water supply is pumped to the boiler, heat energy is added by combustion of fuel, the work output then drives the turbine to generate electricity and finally the steam is then condensed back to water in the condenser
The system operates on a simple Rankine cycle, the water is the working fluid , the rankine cycle is simply a practical Carnot cycle ,the main difference between them is that a pump is used to pressurize liquid instead of gas
4.2 Experimental Procedure
Combined Refrigeration and Mechanical Heat Pump system
Obtain all data for two tests, with the flow of water through the condenser being greater for the second
Find the actual values of COPR and COP HP based on the heat extracted from the water flowing through the evaporator and on the heat added to the water flowing through the condenser, respectively
Comment on the results obtained
Plot data on a p-h chart and find COPR and COPHP based on refrigerant properties
Experiment 2: 1500cc, 4cyl Diesel Engine Test
1. Apply a load to the engine
2. Use the throttle to decrease speed to the lowest stable speed
3. Note speed, torque and fuel consumption
4. Use throttle to increase engine speed in six steps to maximum speed and for each throttle setting, note the speed torque and fuel consumption.
5. Evaluate the Brake Power, Specific Fuel Consumption and Brake Thermal Efficiency corresponding to each engine speed.
6. Plot on a single page graphs of Brake power, Specific Fuel Consumption and Brake thermal efficiency against engine speed
7. Comment on results
Experiment 3 Boiler, Turbine & Steam Power Plant Efficiency Measurement
Carry out at test measuring the salient pressures and temperatures indicated and proceed to find the Boiler Efficiency,, the generator efficiency, , and the cycle efficiency
RESULTS
Combined Refrigeration and Mechanical Heat Pump system
The criterion of performance of a cycle is expressed as the ratio of output/input, which depends upon what is regarded as the output. In the refrigerant the maximum amount of heat extracted from the water in the evaporator Qe is the input . In a heat pump we are concerned to obtain the maximum amount of heat from the condenser water
COPR = and COPHP =
The relation between these two coefficients of performance can be established by applying the first law to give
COPHP = COPR +1
The COPR and COPHP for the first test was 1.324 and 2.366 respectively
The COPR and COPHP for the second test was 2.354 and 2.354 respectively
The p-h chart is in the appendix
Experiment 2: 1500cc, 4cyl Diesel Engine Test
The overall efficiency of a plant is usually expressed as the ratio of input/output , for a engine work is the output and the input is the unit mass of fuel supplied. The net output of an engine is called the brake power and the overall efficiency is termed the brake thermal efficiency nb
mf is the fuel flow kg/s, Cv is the calorific value of fuel j/kg
The specific fuel consumption is used as an alternative criterion of performance, it is defined as the rate of fuel consumption per kW of brake power
|
Test No |
1 |
2 |
3 |
4 |
5 |
6 |
|
Engine speed (RPM) |
1583 |
1747 |
1902 |
2010 |
2013 |
2196 |
|
Brake Power (kw) |
7.627 |
8.2347 |
10.5285 |
11.63 |
11.72 |
13.26 |
|
S.F.C (kg/kW h) |
0.324 |
0.3123 |
0.304 |
0.2714 |
0.3047 |
0.27 |
|
ηb (%) |
0.2545 |
0.263 |
0.2713 |
0.3044 |
0.2709 |
0.305 |
Fig 3 Diesel Engine results
Experiment 3 Boiler, Turbine & Steam Power Plant Efficiency Measurement
As previously mentioned efficiency is the ratio of input/output, the efficiency of the boiler is the work out Q2,4 by work in (fuel) which gives
→ 62.967%
The generator efficiency is the power produced by the work supplied
→74.175%
The cycle efficiency is the amount of work generated by the turbine as a function of the energy supplied by the boiler →0.8%
DISCUSSION
The coefficient of performance for the refrigerant and the heat pump is quite good for the first test and obeys the first law of thermodynamics; the heat given out is more than the heat added. However for the second test the evaporator flow rate was doubled to 100(kg/hr) while the condenser flow rate was reduced from 50 to 40 (kg/hr) this affected the results and the relationship between the Coefficient of performance of the heat pump and the refrigerant does not obey the first law of thermodynamics.
The tests carried out on the diesel engine are correct as you would image that the SFC would reduce when the Brake power is increased. However the brake thermal efficiency doesn't give a very conclusive result in the tests carried out. The results would suggest that the Brake thermal efficiency increases as the brake power increases. I think these results are inaccurate due to the fact in very little increase in SFC, if there were a wider range of results there would be more conclusive answers. For instance the engine speed only increase from 1583 to 2196 rpm, for 6 speed settings, there isn't much of an increase in speed and using analogue instruments this leads to inaccuracies
The final experiment showed that an oil fired burner is quite efficient and so is an electrical generator when compared to an internal combustion engine. However the cycle used to replicate a power plant on the Rankine cycle operates on a very simple basis, it doesn't even re-circulate the condensate. The cycle efficiency is very low which would be right considering the amount of losses in the system
7. CONCLUSIONS
The coefficient of performance of the heat pump and refrigeration cycle obeys the relationship of first law.
The diesel engine operates most efficiently at low engine speeds and the brake thermal efficiency increases with engine speed
The open system for the power plant is not very efficient but does reveals the advances in modern day power stations
8. REFERENCES
Engineering Thermodynamics Work & Heat Transfer 1967 Rogers & Mayhew
ME4516 Thermodynamics 2 Dr Cormac Eason
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